Sodium channel (rnav1.5a) and use thereof

ABSTRACT

The present invention relates to a novel voltage gated sodium channel located in the brain, nucleotides coding for it, vectors and host cells containing the same, transgenic non-human animal capable of expressing the sodium channel, and methods of screening for modulators of the channel such as modulators for use in the treatment of seizures, and conditions related to the limbic system and limbic regions including limbic seizures.

[0001] The present invention relates to a novel voltage gated sodium channel located in the brain, nucleotides coding for it, vectors and host cells containing the same, transgenic non-human animal capable of expressing the sodium channel, and methods of screening for modulators of the channel such as modulators for use in the treatment of seizures, and conditions related to the limbic system and limbic regions including limbic seizures.

[0002] Voltage-gated sodium channels are responsible for the rising phase of the action potential and play a role in a number of conditions related to a mediation of electrical activity in excitable tissues. A number of medicaments are known to act via a sodium channel including a number of cardiac drugs. The sodium channels known in the art include the major cardiac channel Na_(v)1.5 (formerly named H1 or SKM2) and the sensory neurone specific channel Na_(v)1.8 (formerly named SNS/PN3).

[0003] The present invention provides a new sodium channel located in the brain possible only located in the limbic system. More specially, the present invention relates to a voltage gated sodium channel from a rat astrocyte stemcell line called HiB5.

DESCRIPTION OF THE INVENTION

[0004] From a rat astrocyte stemcell line called HiB5 (Renfranz PJ, Cunningham MG & McKay RDG (1991) Region-specific differentiation of the hippocampal stem cell line HiB5 upon implantation into the developing mammalian brain. Cell (66) pp 713-29) a cDNA was cloned and the corresponding voltage gated sodium channel encoded by this sequence was characterised. The cDNA sequence exhibits a near 100% match to the heart specific sodium channel subtype rNa_(v)1.5 (GenBank Accession number M27902; formerly named rH1) but appears to be a splice variant since it lacks 159 nucleotides in position 3238 through 3396 relative to rNa_(v)1.5 coding sequence number. This corresponds to a deletion of 53 amino acids positioned intracellularly between domain II and III in the generally depicted structure of the α-subunit. In addition, a few bases differ between the two sequences but in a way that do not alter the amino acid sequence (see sequence SEQ No. 1).

[0005] By searching the GenBank using sequences surrounding the deleted area, no matches are returned supporting that this sequence is novel. As a control searching with the same sequence including the 159 bases, the subtype rNa_(v)1.5 is returned from GenBank.

[0006] The rNa_(v)1.5 subtype (gene locus: SCN5A) has been found to be expressed in the brain, restricted to the limbic regions (Hartmann, HA, Colom LV, Sutherland ML & Noebels JL (1999) Selective localization of cardiac SCN5A sodium channels in limbic regions of rat brain. Nature neuroscience (2), 7, pp 593-5). However, this work made use of probes for hybridisation and primers for polymerase chain reactions (PCR) that would not recognise the sodium channel according to the present invention, which is a splice variant of the cardiac sodium channel. In line with the nomenclature of this field, the present subtype is called rNa_(v)1.5a (or according to the former nomenclature rH1 A).

[0007] In a clinical perspective it should be noted that some complex seizure disorders are seeded in the limbic region. According to the present invention, conditions related to this region are of special interest, as it is believed that the specific channel according to the present invention plays an important role in this area.

[0008] Furthermore, it is relevant to point out that since rat and man have a very high sequence homology between other subtypes it is expected that a splice variant is also expressed in the limbic region of man. In addition, it has surprisingly been found that known anti-epileptic drugs like Lamotrigine (Lamictal) act more potently on the channel according to the present invention than on the classical brain subtype BIIA (for details, see Example 2) that they are thought to act on, so the anti-epileptic effect might be established via rNa_(v)1.5a.

[0009] Other indications with off spring in the limbic area may also be treated by modulation of rNa_(v)1.5a e.g. attention deficit, depression and other conditions including pain.

[0010] One aspect of the invention therefore provides an isolated nucleotide sequence wherein the sequence is as shown in SEQ No. 1 (rNa_(v)1.5a) or a variant thereof. In a further aspect, the invention relates to an isolated mammalian sodium channel protein encoded by the nucleotide sequence for rNa_(v)1.5a as shown in SEQ No. 1 or a variant thereof. In a still further aspect, the invention relates to an isolated mammalian brain sodium channel protein encoded by the sequence shown herein. In one embodiment, the isolated mammalian sodium channel or nucleotide sequence is isolated from the limbic region of a mammalian. Preferably, the sodium channel of the invention is isolated from the limbic region such as from the hippocampus. The sodium channel may be derived from any mammalian species, preferably from the rat or human. In a further embodiment, the invention relates to an isolated sodium channel derivable from the limbic region of a mammal, e.g. from a rat or human that has an IC₅₀ for Lamotrigine of less than 100 μM, preferably less than 50 μM, such as less than 25 μM, as measured in accordance with Example 2.

[0011] In a further aspect, the invention relates to a sodium channel protein or variant thereof as described above for use in a method for screening for agents acting as modulators on the sodium channel.

[0012] Included within the invention are variants of the sodium channel including fragments, analogues, derivatives and splice variants. The term variant refers to a protein or nucleotide sequence encoding a protein that retains substantially the same biological function or activity as the isolated sodium channel, rNa_(v)1.5a. The identity is preferably 99%, however even identity of 98, 97, 95, 90, and 80% is believed to provide a function comparable with the sodium channel isolated according to the invention. In one embodiment, the invention relates to an isolated nucleotide sequence having at least 98% identity with the sequence as shown in SEQ No. 1.

[0013] Analogues include precursor proteins or fusion proteins. Splice variants refer to a protein produced by the same gene, generated by alternative splicing of mRNA, that contains additions or deletions within the coding region. Splice variants that occur naturally are within the scope of the present invention.

[0014] Fragments also include portions of rNa_(v)1.5a, characterised by structural or functional attributes of the protein. Derivatives include naturally occurring allelic variants. Derivatives may also include non-naturally occurring proteins or fragments. Fragments may be fused or may be comprised within a larger protein or a precursor protein designed for expression in a host.

[0015] The invention also relates to antisense nucleotides or complementary strands to the sequence as disclosed herein as well as RNA, cDNA, genomic DNA and synthetic DNA that encode a mammalian sodium channel isolated from the limbic region of a mammal.

[0016] The nucleotide sequence of the present invention may be used for producing the sodium channel protein or variant thereof by recombinant techniques well known in the art.

[0017] Accordingly, in a further embodiment the present invention relates to a recombinant construct comprising the nucleotide sequence as described herein or variants thereof, an expression vector, such as a plasmid into which the sequence of the invention has been inserted, preferably, a promotor is operably linked to the sequence. Suitable vectors and promoters are known to the skilled person. In one embodiment, the invention relates to a recombinant polynucleotide comprising the nucleotide sequence as shown in SEQ No. 1 or a variant thereof. In a further embodiment, the invention relates to a vector, e.g. plasmid comprising a nucleotide sequence as described above. In a special embodiment, the invention relates to a vector wherein the nucleotide sequence is labelled with a detectable moiety. In a still further embodiment, the invention provides a host cell for example a higher eukayotic cell such as a mammalian cell or a lower eukaryotic cell. In a special embodiment, the invention relates to a host cell transfected with a vector as described above. In a further embodiment, the invention provides an immortalised mammalian cell line comprising the sequence as shown in SEQ No. 1 or a variant thereof.

[0018] The term isolate according to the present invention means that the material is removed from its original environment. The proteins and nucleotide sequences according to the present invention are also preferably provided in purified form and preferably to at least 50% purity, such as at least 75% purity, more preferred 90% purity, most preferred at least 95% purity such as 98% purity.

[0019] In a further aspect, the invention relates to a transgenic non-human animal comprising a diploid genome comprising a transgene including the sequence encoding as shown in SEQ No. 1 or a variant thereof and wherein the transgene is expressed to produce a sodium channel. In one embodiment, said sodium channel is expressed in an amount sufficient to be detectable in a brain homogenate of the transgenic animal. In a second embodiment, the animal is murine. In a further embodiment, the transgene is non-homologously integrated. In a still further embodiment, the expression of the sodium channel is under the control of a promotor sequence different from the promotor sequence controlling the transcription of the encoding sequence for the sodium channel.

[0020] In another embodiment, the present invention relates antibodies specific for the rNa_(v)1.5a. The antibody may be mono or polyclonal, may be intact antibody molecules or fragments containing the active binding region of the antibody. The antibody according to the present invention includes antibodies produced by well known techniques in the art. The antibodies of the invention may also be used for purifying the sodium channel of the present invention. In one embodiment, the invention relates to an antibody or fragment thereof which recognises and/or binds to a sodium channel encoded by the sequence as shown in SEQ No. 1 or a variant thereof. In another embodiment, the invention relates to an antibody obtained by means of an immune response to exposure to a substantially purified sodium channel encoded by the sequence as shown in SEQ No. 1 or a variant thereof. In a special embodiment, the antibody is monoclonal or polyclonal.

[0021] In an interesting aspect of the invention the sodium channel is used to identify or screen modulators of the channel. Different techniques known in the art may be utilised in this respect including patch clamp technology.

[0022] Thus, in a further aspect, the invention relates to a method for the identification of a modulator of a sodium channel encoded by the sequence as shown in SEQ No. 1 or a variant thereof comprising contacting said channel with a test compound and detecting activity or inactivity of said channel. In one embodiment, the invention relates to a method of assaying test compounds which modulate sodium flux comprising expressing a protein or variant thereof encoded by the sequence as shown in SEQ No. 1 or a variant thereof. In a special embodiment, the method involves utilising patch clamp technology. The test methods according to the present invention are very useful for identifying pharmaceutically active compounds useful for disease and condition of the brain. The new brain specific sodium channel provides an important tool for identifying components, known or developed in the future, which may have a selective effect on the brain, and especially on the limbic system. The diseases or conditions may in addition to convulsions include panic disorders, hyperactivity disorders, depression, obsessive compulsive disorders, dementia, memory deficits, attention deficit, obesity, anxiety, eating disorders, drug addiction and misuse, altered sexual drive. Parkinson's disease and Alzheimer's disease may also be treated with compounds having an effect on the sodium channel according to the invention. Furthermore, conditions related to visceral responses originating to the limbic system may also be prevented or treated by use of medicaments capable of modulate the sodium channel according to the present invention. Such visceral symptoms may include respiration, and gastrointestinal movements and secretion.

[0023] In a still further aspect, the invention relates to a method and diagnostic kit for identifying a disease or condition wherein the function of the sodium channel is altered. Such method or kit may involve a labelled antibody to the sodium channel of the present invention.

[0024] In a further aspect, the invention relates to a compound identified by any of the methods as described above. In one embodiment, the compound has an IC₅₀ which is at least 10 times smaller than the IC₅₀ for rNa_(v)1.2a measured as disclosed in Example 2.

Example 1

[0025] Preparation

[0026] To elucidate the subtype present in HiB5 cells after having detected specific sodium current by means of electrophysiological measurements, Reverse transcriptase-polymerase chain reaction (RT-PCR) was set up. For primer design alignment of five different subtypes was made including rNa_(v)1.1 (formely named rBI), rNa_(v)1.2a (formerly named rBIIA), rNa_(v)1.3 (formerly named rBIII), rNa_(v)1.4 (formerly named rSkMI), rNa_(v)1.5 and rNa_(v)1.7 (formerly named rPN1). In regions of high homology, 6 sense and 6 antisense degenerated oligos (15-23mers) where designed and paired to amplify regions thas displayed less homology in the alignment. Fragments would range from 500-1000 basepairs. Template cDNA was made by RT-PCR with random hexamers on mRNA extracted using “mRNA capture Kit” from Boehringer Mannheim.

[0027] The RT-reaction was performed with Superscript II reverse transcriptase at 42° C. for 50 min. The PCR was performed with 1 unit DNApolymerase constituting Taq:Pwo in the ratio 9:1, 200 μM dNTP and 500 nM primer. The mix was melted at 94° C. for 1:30 and the cycled 15 times at 94° C. for 0:30, 45° C. for 0:30 and 72° C. for 1:30 followed by 10 times at 94° C. for 0:30, 45° C. for 0:30 and 72° C. for 2:30. Fragments were completed at 72° C. for 5:00.

[0028] Four fragments were obtained using this approach. Search against GenBank gave a match to rNa_(v)1.5. Four sets of rNa_(v)1.5-specific primers where then designed to amplify 4 overlapping fragments. The template for these reactions was cDNA made from total RNA (“RNeasy mini kit”, Qiagen) using oligodT as primers for the first strand. Another RT-reaction was performed using random hexamers. Both were with Superscript II reverse transcriptase at 42° C. for 50 min. PCR amplification for the four fragments were carried out employing nested primer pairs to ensure specificity.

[0029] The first PCR was performed as: 94° C. for 1:30 and the cycled 15 times at 94° C. for 0:30, 45° C. for 0:30 and 72° C. for 1:30 followed by 10 times at 94° C. for 0:30, 45° C. for 0:30 and 72° C. for 2:30. Fragments were completed at 72° C. for 5:00.

[0030] The nested PCR was performed as: 94° C. for 2:30 and the cycled 10 times at 94° C. for 0:45, 55° C. for 0:45 and 72° C. for 3:00 followed by 15 times at 94° C. for 0:45, 55° C. for 0:45 and 72° C. for 4:00. Fragments were completed at 72° C. for 5:00.

[0031] All fragments were cloned in an in-house vector called pSwaS derived from PCR-script from Invitrogen. Plasmids were amplified in the E. coli strain XL1-B which were grown on agar plates over night at 37° C. Plasmid amplification was carried out using standard LB media for culture and purifications kit from Qiagen for retrievement of plasmid DNA. Integrety of the plasmid was checked with restriction enzymes and gel electrophoresis.

[0032] The four fragments positioned in each pSwaS vector were excised with restriction enzymes in overlapping regions and ligated to form to vectors, one containing the 5′ half and the other containing the 3′ half. The vectors were amplified in XL1-B and checked by restriction enzyme cleavage. The 5′ half was excised and ligated into the vector containing the 3′ half to give the entire coding sequence.

[0033] This plasmid was amplified in XL1-B. Following this the coding sequence was excised and subcloned into an in-house expression vector pNS1z. This plasmid was amplified in the E. coli strain SURE which has a lower copy number than XL1-B and a lower tendency to rearrangement of repetitive eukaryotic sequences like sodium channels.

[0034] Purified plasmid was used for transfecting HEK293 cell and Zeocin was added to the growth medium for selection of stabile clones. Stabile clones from the primary culture flask were transferred to deepwell plates and when confluency in the wells the cells were transferred to T25 culture flasks. Following 5 passages, Zeocin was omitted from the medium and expression of the channel was monitored electrophysiologically.

EXAMPLE 2

[0035] Comparison of the Inhibitory Effect of Lamotrigine on the Classical Brain Type rNa_(v)1.2a and the sodium channel expressed endogenously in HiB5 Cells (rNa_(v)1.5a)

[0036] Experiments were performed as whole-cell recordings in physiological saline with stimulation of the sodium channel every 5 second by stepping from a holding potential (e.g. −90 mV) to −10 mV and monitoration of the peak current as a function of the concentration of Lamotrigine added to the recording chamber.

[0037] This gave rise to these values:

[0038] IC₅₀=500 μM for rNa_(v)1.2a

[0039] IC₅₀=20 μM for rNa_(v)1.5a

[0040] Recovery from inactivation is slowed by Lamotrigine more pronounced on HiB5 cells than on rNa_(v)1.2a. This is a parameter of use-dependent mode of action that is desirable in treating epilepsy and therefore supports the idea that the modulation of rNa_(v)1.5a is a key factor in treating epilepsy.

1 1 1 5901 DNA Rattus norvegicus 1 atggcaaacc tcctgttacc tcggggcacc agcagcttcc gtaggttcac ccgggagtca 60 ctggcggcca tcgagaagcg aatggctgaa aagcaagccc gaggaggttc ggccacctca 120 caggagagcc gtgagggcct gcaggaggag gaggctcccc ggccccagct ggacctacag 180 gcctccaaaa agctgccaga tctctatggc aacccacccc gagagctcat cggggagccc 240 ctggaagacc tggacccttt ctatagtacc cagaagacct tcatcgtgct gaataagggc 300 aaaaccatct tccggttcag tgccaccaat gccttgtatg tcctcagccc cttccacccc 360 gtgcgccgag cggccgtgaa gatcctggta cactcgctct ttagcatgct catcatgtgc 420 accatcctga ccaactgcgt gttcatggcc cagcacgacc ctccgccttg gaccaaatat 480 gttgagtaca ccttcactgc catctacacc tttgagtctc tggtcaagat tctagctcga 540 ggcttctgcc tgcatgcatt caccttcctt cgggacccgt ggaactggct agacttcagt 600 gtgatcgtca tggcatacac aactgaattt gtggacctgg gcaatgtctc agccttacgc 660 accttccgag tcctccgggc cctgaaaact atatcggtca tttcaggcct gaagaccatc 720 gtgggagccc taatccagtc tgtgaagaaa ctggccgatg tgatggtcct cactgtcttc 780 tgcctcagtg tctttgccct cattggcctg cagctcttca tgggcaacct gaggcacaag 840 tgtgtgcgta acttcaccga gctcaatggc accaatggtt ccgtggaggc cgacggccta 900 gtctggaact ccctggacgt ctacctcaat gacccagcca attacctgct caagaatggc 960 accacggatg tgttactatg tgggaacagc tctgatgccg ggacatgccc tgagggctat 1020 cggtgcctga aggcaggtga gaacccagac cacggttaca ccagcttcga ctccttcgcc 1080 tgggccttcc ttgcactctt ccgcctgatg acacaggact gctgggaacg cctataccag 1140 cagaccctga ggtccgcagg aaagatctac atgatcttct tcatgctcgt catctttctg 1200 ggctccttct acctggtgaa cttgatcctg gctgtggtgg ccatggccta cgaggagcaa 1260 aaccaagcca ccatcgccga gacggaagag aaggagaagc gcttccagga ggccatggag 1320 atgctcaaga aggaacacga ggctctcacc atcaggggtg tggataccgt gtcccgtagc 1380 tctctggaga tgtctccttt ggccccagta accaaccatg agagaaagag caaaaggagg 1440 aaacgactat cttcagggac agaggatggt ggggatgaca ggctccccaa gtcggactca 1500 gaagatggtc ccagagcatt gaatcagctc agcctcaccc atgggctcag ccggacatcc 1560 atgaggcccc gctcgagccg agggagcatt ttcacgttcc gaagacggga ccaaggctct 1620 gaggcggact tcgcagatga cgagaacagc actgcggggg agagcgagag ccaccgcaca 1680 tcgctgctgg taccctggcc cctgcgccat cccagcgccc aaggacagcc cggccctgga 1740 gcctcagctc ccggttacgt tctcaatggc aaaaggaaca gcaccgtgga ctgcaatggg 1800 gtggtttcct tgctgggggc aggtgacgca gaggccacct ccccagggag ctaccttctc 1860 cgccctatgg tgctggaccg acccccagac acgaccactc cgtcagagga gcccggtggg 1920 ccccagatgc tgacacctca ggctccgtgt gcagatggtt ttgaggagcc cggagcacgg 1980 caacgggcac tcagcgctgt cagtgtcctc accagtgccc tggaagagtt ggaggagtcc 2040 catcggaagt gtccaccatg ctggaaccgc tttgcccagc actacctcat ctgggagtgc 2100 tgtccactct ggatgtccat caagcagaag gtgaagtttg tggtcatgga cccatttgcc 2160 gacctcacta tcaccatgtg catcgtgctc aatacgctct tcatggctct ggagcattac 2220 aacatgacgg cagagtttga ggagatgctg caggtcggaa acctggtctt cacgggaatc 2280 ttcacagcgg agatgacctt caagatcatc gcccttgacc cctactacta cttccagcag 2340 ggctggaata tcttcgacag catcatcgtc atcctcagtc tcatggagct ggggctgtcc 2400 cgcatgggca acttgtctgt gctacgttcc ttccgtctgc tgcgggtctt caagctggcc 2460 aagtcctggc ccaccctgaa cacgctcatc aagatcatcg ggaactccgt gggcgccctg 2520 gggaacctga ccctggtgct ggccatcatc gtcttcatct tcgccgtggt gggcatgcag 2580 ctcttcggca agaactactc ggagctgagg caccgcatca gcgactccgg cctgctgccc 2640 cgctggcaca tgatggactt tttccacgcc ttcctcatca tcttccgcat cctctgtggg 2700 gagtggatcg agaccatgtg ggactgcatg gaggtgtctg ggcagtcgct gtgcttgctg 2760 gtcttcctgc tcgtcatggt cattggcaac cttgtggtcc tgaatctctt cttggccttg 2820 ctgctcagct ccttcagcgc agacaacctc acagcccctg acgaggatgg ggagatgaac 2880 aacctccagc tggccctggc tcgcatccag aggggcctgc gctttgtcaa gcggaccacc 2940 tgggacttct gctgcgggat cctgcggcgg cgacctaaga agcccgcggc tcttgccacc 3000 cacagccagc tgcccagctg tatcaccgcc cccaggtccc caccaccccc agaggtggag 3060 aaggtgcccc cagcccgcaa ggaaacacga ttcgaggagg acaagcgacc cggccagggc 3120 acccctgggg attcggagcc tgtgtgtgtg cccatcgccg tggctgagtc agacactgaa 3180 gaccaggaag aggatgaaga gaacagcctt ggcacagagg aagagtccag caaacagacc 3240 cctgaggaca gttactccga gggcagcaca gctgacatga ccaacaccgc cgacctcctg 3300 gagcaaatcc cagaccttgg tgaggacgtc aaggacccag aggactgctt tactgaaggc 3360 tgcgtccgac gctgtccctg ctgcatggta gacacaaccc agtccccagg gaaggtctgg 3420 tggcgattgc gcaagacctg ctaccgcatc gtggagcaca gctggttcga gactttcatc 3480 atcttcatga tcctgctcag cagtggagcg ctggccttcg aggacatcta cctggaggag 3540 cggaagacca tcaaggttct gctggagtac gcggacaaga tgttcaccta cgtctttgtg 3600 ttggagatgc tgctcaagtg ggtggcctac ggcttcaaga agtacttcac caacgcctgg 3660 tgctggctgg acttcctgat tgtggacgtc tcgctggtca gcctcgtggc aaacacctta 3720 ggcttcgccg aaatgggtcc catcaagtca ctgaggacac tgcgtgcact tcgacccctg 3780 agggccttgt cgagatttga gggcatgcgg gtggtggtca atgcgctggt gggcgccatc 3840 ccctccatca tgaacgtcct cctcgtctgc ctcatcttct ggctcatctt cagcatcatg 3900 ggcgtgaacc tcttcgccgg gaagttcggt aggtgcatca accagacaga aggggacctg 3960 cctctgaact acaccatcgt gaacaacaag agtgagtgcg agtccttcaa cgtgaccgga 4020 gagttgtact ggaccaaggt gaaggtcaac tttgacaacg tgggagccgg gtacctggcc 4080 ctcctgcagg tggcgacatt taaaggctgg atggacatca tgtatgcggc tgtggactcc 4140 agagggtatg aggagcagcc gcagtgggaa gacaacctct acatgtacat ctactttgtc 4200 gtcttcatca tcttcggctc cttcttcacc ctcaacctct tcatcggtgt catcattgac 4260 aacttcaacc agcagaagaa aaagttaggg ggccaggata tcttcatgac ggaggagcag 4320 aagaagtact acaatgccat gaagaagctg ggctccaaga aaccccagaa gcccatccca 4380 cggcccttga acaagtacca gggtttcata ttcgacattg tgaccaagca ggccttcgat 4440 gtcaccatca tgttcctcat ctgtttgaac atggtgacca tgatggtgga gacagatgac 4500 cagagccctg agaaggtcaa catcttggcc aagatcaacc tgctcttcgt ggccatcttc 4560 acaggcgagt gtattgtcaa gatggctgcc ctgcgccact attacttcac caacagctgg 4620 aacatcttcg actttgtggt ggtcatcctc tccattgttg gcactgtcct ctccgacatc 4680 atccagaagt acttcttctc cccgacactc ttccgggtca tccgtctggc caggatcggc 4740 cgcatcctca ggctgatccg cggagccaag gggattcgca cgctgctctt cgccctcatg 4800 atgtccctgc ccgccctctt caacatcggc ctcctcctct tcctcgtcat gttcatctac 4860 tccatcttcg gcatggccaa cttcgcttac gtcaagtggg aggccggcat cgatgacatg 4920 ttcaacttcc agaccttcgc caacagcatg ctgtgcctgt tccagatcac cacatcagcc 4980 ggctgggacg gcctcctcag ccccatcctc aacacggggc ctccctactg cgaccccaac 5040 ctgcccaaca gcaacggctc ccgggggaac tgtgggagcc cggcggtggg catcctcttc 5100 ttcaccacct acatcatcat ctccttcctc atcgtggtca acatgtacat cgccatcatc 5160 ctcgagaact tcagcgtggc caccgaggag agcacagagc ccctgagcga ggacgacttc 5220 gacatgttct atgagatctg ggagaagttc gacccggagg ccacccagtt cattgagtat 5280 ctggccctgt ccgactttgc agatgccttg tctgagccgc tccgcatcgc caaacccaac 5340 cagataagcc tcatcaacat ggatctgccc atggtgagcg gagaccgtat ccactgtatg 5400 gacatactgt tcgctttcac caagagggtg ctcggcgagt ctggggagat ggatgccctg 5460 aagatccaga tggaggagaa gttcatggcg gccaaccctt ccaagatctc ctacgagccc 5520 atcaccacca ccctgaggag aaagcacgag gaggtgtcgg ccacggtcat ccagcgtgcc 5580 ttccggaggc acctgctgca gcgctcggtg aagcatgcct cctttctctt ccgccagcaa 5640 gcgggcggca gtggcctctc cgacgaggat gcccctgagc gggagggcct catcgcctac 5700 atgatgaatg ggaacttctc tcggcgcagt gctccgctct ccagctcctc catctcctcc 5760 acgtccttcc ccccgtccta cgacagcgtc acgagagcca ccagtgataa cctcccggtg 5820 cgtgcgtctg actatagccg cagcgaagat cttgcagact tccctccatc tccagatagg 5880 gaccgagagt ctatcgtgtg a 5901 

1. An isolated nucleotide sequence wherein the sequence is as shown in SEQ No. 1 (rNa_(v)1.5a) or a variant thereof.
 2. An isolated mammalian sodium channel protein encoded by the nucleotide sequence for rNa_(v)1.5a as shown in SEQ No. 1 or a variant thereof.
 3. An isolated mammalian sodium channel or nucleotide sequence according to any of claims 1 and 2 isolated from the limbic region of a mammalian.
 4. A sodium channel protein or variant thereof according to claim 2 or 3 for use in a method for screening for agents acting as modulators on the sodium channel.
 5. A recombinant polynucleotide comprising the nucleotide sequence according to claim 1 or a variant thereof.
 6. An isolated sodium channel derivable from the limbic region of a mammal, e.g. from a rat or human that has an IC₅₀ for Lamotrigine of less than 100 μM, preferably less than 50 μM, such as less than 25 μM, as measured in accordance with Example
 2. 7. An isolated nucleotide sequence having at least 98% identity with the sequence according to claim
 1. 8. A vector, e.g. plasmid comprising a nucleotide sequence according to any of claims 1 and
 7. 9. A vector according to claim 8 wherein the nucleotide sequence is labelled with a detectable moiety.
 10. A host cell transfected with a vector according to any of claims 8 and
 9. 11. An immortalised mammalian cell line comprising the sequence or variant thereof according to claim
 1. 12. A transgenic non-human animal comprising a diploid genome comprising a transgene including the sequence encoding as disclosed in claim 1 and wherein the transgene is expressed to produce a sodium channel.
 13. A transgenic non-human animal according to claim 12 wherein said sodium channel is expressed in an amount sufficient to be detectable in a brain homogenate of the transgenic animal.
 14. A transgenic animal according to any of claims 12 or 13 wherein the animal is murine.
 15. A transgenic animal according to any of claims 12-14 wherein the transgene is non-homologously integrated.
 16. A transgenic animal according to any of claims 12-15 wherein the expression of the sodium channel is under the control of a promotor sequence different from the promotor sequence controlling the transcription of the encoding sequence for the sodium channel.
 17. An antibody or fragment thereof that recognises and/or binds to a sodium channel or a variant thereof encoded by the sequence according to claim
 1. 18. An antibody obtained by means of an immune response to exposure to a substantially purified sodium channel encoded by the sequence according to claim
 1. 19. An antibody according to claim 18 that is monoclonal or polyclonal.
 20. A method for the identification of a modulator of a sodium channel according to claim 1 or a variant thereof comprising contacting said channel with a test compound and detecting activity or inactivity of said channel.
 21. A method of assaying test compounds that modulate sodium flux comprising expressing a protein or variant thereof encoded by the sequence according to claim
 1. 22. A method according to any of claims 20 or 21 utilising patch clamp technology.
 23. A compound identified by any of the methods according to claims 20-22.
 24. A compound according to claim 23 having an IC₅₀ that is at least 10 times smaller than the IC₅₀ for rNa_(v)1.2a measured as disclosed in Example
 2. 